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12:03 AM
It's transcendental, but it's a disguised version of an algebraic equation. When I referred to "trivial circumstances," I meant starting with an algebraic equation and "artificially" turning it into a transcendental one. Using $e^{\ln x} = x$ is what you did.
 
So do you think it's a trivial equation
 
I think that's a trivial way of making up transcendental equations you can solve, yes.
 
but couldn't you still define an algebraic variety from these equations
 
Yes, but that's the sort of trivial situation I was referring to earlier. I don't think this is an interesting question.
OK, I need to get going.
 
@TedShifrin I understood that. But then how to get that equation
 
12:20 AM
@TedShifrin inb4 schemes
 
12:30 AM
topology $\neq$ topography
$\qed$
where is this
 
@Fawad: Find the equation of the line through $z_1$ and $z_2$ and then think.
 
Hey everyone!
 
Heya @Daminark
 
How's it going?
 
12:52 AM
Hello
 
Hey Paul!
 
whats up with you @Daminark
 
It's going, just finally finished section 1.2 in GP, lol
 
0
Q: Why is the equality $|L\backslash M_H|=|L|-|M_H|$true? Why $|L\backslash M_H|\ge|U|$?

Math geek Doubt on underline 1 What do you mean by the edge subgraph of $G$ defined by $L$?IsIt a subgraph with all edges from $L$? Doubt on underline 2 Why is the equality $|L \backslash M_H|=|L|-|M_H|$true? Why $|L\backslash M_H|\ge|U|$? Can you please explain?

pl;\
please help me.
 
Nice! I just landed with my dad in Chicago
 
1:05 AM
Is school just starting, or where you traveling?
 
School's gonna start on Monday
Here a few days early to get things set up and all
 
1:35 AM
Sick.
 
1:54 AM
@TedShifrin solving argument I get something like arctan(a/b)=+pi/2 . Now for b=0 I got circle equation . Also writing a>0 gives me second condition. Although I didn’t understand what wanted me to do
 
2:37 AM
@Daminark lol nerd starting school
 
2:48 AM
I love $\phi$ and it's properties
the g-ratio
 
3:39 AM
@EricSilva no u
 
Actually, let's formalise it:
Consider a group isomorphic to $\Bbb{Z}/2$
Then define a map $\infty$ such that it maps $\Bbb{Z}/2$ to $\{e\} \times \{e'\}$
Then we have: $\infty (\Bbb{Z}/2) = \{e\}$ and $(\Bbb{Z}/2)\infty = \{e'\}$
The "No counter" is thus realised by the orbit of any trivial group has a period of 1
and all "no U"s are multiplying by an inverse element
 
@Daminark What classes are you doing?
 
4:10 AM
Does someone know what is TA'ing? Is teaching assisting? Or what does it mean?
 
@Isabella A TA is a teaching assistant, you are correct.
 
ok thanks
 
I have a question. Please confirm if my understanding is correct. If A and B are 2 events. And if A is a super-set of B (imagine a diagram B inside A). Then it means if B happens A also automatically happens. Correct so far ? So the probability of (A U B) is equal to probability of B
 
@Daminark shit lol u got me
 
4:30 AM
@AlexClark representation theory, probably undergrad but maybe grad if the students who told me the professor no longer as ridiculously harsh as he used to be are right, AT, AG, and a reading course in NT (not sure what exactly yet, will discuss with my professor soon)
Eh probably undergrad in any event, two grad classes + grad apps would be rough times
 
4:47 AM
Is homomorphisms necessarily onto?
*are
I thought this was the case but I am getting confused
 
What's your context? Generally, no.
 
I was getting confused when they talked about homorphisms from Z30 -> Z20.
So being onto is an additional property not a defining property
 
Right.
In fact, if you ever have a subgroup H of a group G, you automatically get a homomorphism from H into G (why?), which of course is only onto when H = G.
(I'm guessing you're working with groups and not rings or other algebraic structures for that matter.)
 
Yes I am only on groups for now.
I know that a normal subgroup gives a normal subgroup in the image set. Is the opposite true.
 
How would you state the opposite of that?
 
5:02 AM
Like if I have a subgroup of the image which is normal can I claim that preimage of this set is normal?
 
Ok thanks a lot :)
 
Can you clarify something, though?
Suppose H is the given subgroup of the image K, and G is the codomain. When you say H 'is normal', do you mean H is normal as a subgroup of K or as a subgroup of G?
 
@Daminark Sounds like pretty much the best selection of courses to have in one semester :D
 
5:31 AM
@KarlKronenfeld in my case the homomorphism was onto so the codomain and image were the same.
So what difference does it make? I imagine when you said yes you thought H was normal to K and not G.
 
Suppose, as you figured I was thinking, that the statement is valid for the case of H being normal as a subgroup of K. Can you handle the other case, when H is normal as a subgroup of G?
 
If H is normal in G then H should also be normal in K, right? As K is a subgroup of G that completely contains H.
 
Of course. So, answering the question, what difference does it make: none.
I mean, you might try finding an example where H is normal in K but not G; so one formulation is stronger than the other. But neither is incorrect.
If that was too easy, try finding one where K is normal in G as well.
 
5:57 AM
Ok that makes sense.
 
6:25 AM
If I have a subgroup and a normal subgroup is the product of them a subgroup?
 
@PiyushDivyanakar Yes
 
Product as in $KH = kh \forall k\in K , h \in H$
But not when K and H are just subgroups?
 
not in general, though it can happen that it is a subgroup with neither of the original subgroups being normal
 
6:40 AM
Can you give me a hint to prove the second isomorphism theorem?
I have that K is a subgroup and H is a normal subgroup, I have to show that $K/K \cap H$ is isomorphic to $KH/H$.
 
I have posted the problem with the solution. Can someone please explain this problem? It will be a big help!
 
@PiyushDivyanakar Try to find a map from $K$ to $KH/H$ with the appropriate kernel
(picking a map is actually easy, as there is basically no choice but the right one)
 
7:04 AM
Is it $k \to kH$?
And the kernal of this will be $ K \cap H $
So it follows from first isomorphism theorem?
 
7:19 AM
right
 
8:19 AM
Mornin' all
 
8:31 AM
Hi @ÍgjøgnumMeg
 
9:07 AM
@Alessandro how's it going? Started yet?
 
Nope, I'm packing, I'll move to Bonn on Sunday
 
Ahh nice, good luck
I just spent 270 pounds to register for the German test I'm taking in November :(
 
Thanks!
Standardized language tests are crazy expensive
 
yeahhh, it's the highest level one you can take in the CEF so at least I won't have to pay for another
lol
 
9:32 AM
A meagre consolation
Luckily they accepted the English exam I did as part of my courses for Bonn rather than requiring a toefl or something like that
 
hahah nice
 
Indeed, they accept more or less any proof that you can speak English
 
9:58 AM
that's cool, Heidelberg and Frankfurt basically denied me entry because I didn~t have any proof of my german capabilities
 
10:25 AM
@AlessandroCodenotti: Do you mind explaining a thing or two in your answer?
I don't get how you obtain $\bar{y}$ from $y_{2n}=x_{2n}$ and $y_{2n-1}=2^nx_{2n}$. If we start by plugging in $n=0$ in $y_{2n-1}$ to get $y_1$ we get $2^0x_{2\cdot0}=x_0$, but we have $y_1=x_1$. The rest of the sequence is off in a similar way. I'm also curious as of why $\overline{c_{00}}=\ell^1. I'm not that good with topology, so the closure of a set often seems a bit mysterious to me.
 
is any1 here
 
10:40 AM
@AlessandroCodenotti Ohh, are you in Bonn now?
(and did I know this and just forgot?)
 
11:00 AM
If $c\in E$ is defined as $c_{2n-1}=0$ for all $n\geq1$ and $c_{2n}=\frac{1}{2^n}$ for all $n\geq1$, and, again, $X=\{x=(x_n)_{n\geq1}\in E:x_{2n}=0\forall n\geq1\}$. Then isn't $c\cap X=\emptyset$? I was to show that $c\notin X+Y$, but then I am to construct $Z=X-c$ and check that $Z\cap Y=\emptyset$ where $Y=\{y=(y_n)_{n\geq1}\in E:y_{2n}=\frac{1}{2^n}y_{2n-1}\forall n\geq 1\}$. I'm a bit lost.
@mercio: Do you have any idea?
 
$c \cap X$ is not a sensible object to consider
$c$ is an element of $E$
$X$ is a subset of $E$
so what you can do is ask if $c \in X$
and in fact, c $\notin X$
also I have no idea what you are trying to do
is it showing $X$ is closed ?
 
11:16 AM
If I extend the scalars of a central simple algebra $A$ over a field $F$ to some ring $R$, can it become non-central?
 
I'm trying to do part 2 and 3.
It's from Brezies book on functional analysis. The chapter on Hahn-Banach.
 
you have to show that $c$ cannot be obtained as a sum of an element $x \in X$ and an element $y \in Y$
 
Okay. So it's wrong to think that if $c$ isn't in either $X$ or $Y$, then it isn't in $X+Y$. I guess that's just analogous as to sets of numbers. If $c$ isn't in $X=\{1,2,3\}$ and not in $Y=\{4,5,6\}$, then it still might be in $X+Y$.
 
@abenthy isn't this equivalent to asking if the center of $M_n(R)$ is $R I_n$ ?
@OskarTegby yes, a sum of vector spaces is not the same thing as a reunion of those spaces
 
Right.
I'm a bit unsure as of how to work here.
 
11:21 AM
you probably have to suppose $c = x+y$ with $x\in X$ and $y \in Y$ and try to get a contradiction
 
Okay.
 
@mercio Mhmm, but not every central simple algebra over $F$ must be a matrix algebra, must it? For example, if you tensor the real hamilton quaternions over some ring extension of $\mathbb{R}$, is it still central?
Ah, maybe you use the fact that every CSA is isomorphic to a matrix algebra over some field extension.
 
hmm well you said "central simple algebra over a filed $F$" but that hmm
you're right it's not necessarily $F$
like the quaternions
 
I still don't get it. If $c\notin X$, how can we then consider $Z=X-c$?
 
what do you think that notation means ?
(sorry abenthy, I am not familiar enough in your subject to answer you)
 
11:31 AM
$Z=X-c=X\backslash\{c\}$
 
@mercio No problems, thanks for thinking about it.
 
well no here, $-$ is like $+$, it means $\{x-c ; x \in X\}$
 
That makes quite a difference. :)
Thanks for pointing that out!
 
indeed
have you showed that $X \cap Y = \{ 0 \}$ ?
 
No.
 
11:33 AM
and that if $z \in X + Y$ then there is a unique $x \in X$ and a unique $y \in Y$ such that $z = x+y$ ?
because then it kinda helps to see why the $c$ is not in $X+Y$
because really you have only one choice for $x$ and $y$
 
Okay.
 
I think I found a solution. Seems like if $A$ is an algebra over a field $F$ and $F \subset R$ a ring extension, then $Z(A \otimes_F R) = Z(A) \otimes_F R$. From: mathoverflow.net/questions/137584/…
Interestingly, one needs that $F$ is a field.
 
You mean that $X\cap Y=\emptyset$ and that if $z\in X+Y$ then $x\in X$ and $y\in Y$ are unique such that $z=x+y$ means $c$ can't be in $X+Y$ as there's only one choice for $x$ and $y$?
 
no
I mean to say that when you assume that $c$ is in $X+Y$ you don't know at first what $x$ and $y$ are possible in the decompositions of $c$ as $x+y$.
But it turns out that there is only one possibility, so you can deduce from there that $x$ has to be a certain sequence, and that $y$ has to be a certain sequence.
And form there you just have to realize that $x$ actually is not in $X$, or $y$ actually is not in $Y$
and it's a contradiction
 
hii
 
11:39 AM
hi
 
I don't see that last step.
 
maybe you have to see the middle step first
if $c = x+y$, what's $x_7,x_8,y_7$ and $y_8$ ?
wait are you on question 2 or on question 3 ?
 
2
I will do both, eventually.
$x_8=0$ as $x_{2n}=0$, and $y_8=\frac{1}{2^4}y_7$ as $y_{2n}=\frac{1}{2^n}y_{2n-1}$, but I don't know what $x_7$ or $y_7$ are.
 
what are $c_7$ and $c_8$ ?
 
$c_7=x_7+y_7$ and $c_8=x_8+y_8$
 
11:46 AM
but you have been given a particular sequence $c$
 
oh
so $x_8$ and $y_8$ were right?
 
you haven't given the value for $y_8$ but $x_8$ is right, yes
$x_8 = 0$
 
How do you mean that we were given a particular sequence for $c$. Didn't we just write that $c=x+y$ where $x\in X$ and $y\in Y$ can be any elements?
 
do you remember when the exercise says "let $c \in E$ be defined by ...." then asks you to prove stuff about $c$ ?
it did not define $c$ by saying it was in $X+Y$
 
Oh! Yeah. Damn it! I forgot. How silly of me.
Exactly.
 
11:52 AM
hopefully you will need to look at the definition of what $c$ actually is in order to answer their question, or else you sohuld be really worried
 
Yes.
I'm a bit lost on why you're considering $x_7$, $x_8$, $y_7$, and $y_8$.
 
I am trying to make you realize how much things you can find out aobut the values of $x$ and $y$ in general
 
Okay.
 
if you are able to determine $x_n$ and $y_n$ for $n=7,8$ it should give you hope to do the same for all $n$
 
We just need to show that there's no way to combine elements of $X$ and $Y$ to get $c_{2n-1}=0$ and $c_{2n}=\frac{1}{2^n}$.
Okay. Yeah!
Don't we need to know what $y_1$ is though?
We can't know any $x_{2n-1}$ as it isn't specified in the set.
 
11:59 AM
With first order inhomogenous ode why don’t we have to have the sum of a particular and complimentary to form the general so,union?
like we do with second order?
 
Don't we just know things about the even numbered elements in $X$ and $Y$?
 
Hello. Does anyone have knowledge of type 2 fuzzy sets?
 
no you know lots of other things
write everything you know about $c_7,c_8,x_7,x_8,y_7,y_8$
 
Well hmmm I don't want to get banned for not worshipping everything the internet says, all the time, according to the rules of what is correc but I find this interesting and want to discuss
$$\sum _{i=\pi (n ) +1}^{\pi (n+1 ) } \sum _{j=1}^{ {\bigl\lfloor\frac {\ln ( n ) }{\ln ( p_{{i}} ) }}\bigr\rfloor +1} \Bigl\lfloor {\frac {n}{{p_{{i}}}^{j}}} \Bigr\rfloor \ln ( p_{{i}}) -\sum _{i=\pi (n ) +1}^{\pi (n+1 )} \sum _{j=1}^{\pi ( {\lfloor\frac {n}{i}} \rfloor ) } \Bigl\lfloor\frac{ \ln ( {\frac {n}{i}} ) }{ \ln
p_{{j}} } \Bigr\rfloor \ln ( p_{{j}}
) = 0 \not\Rightarrow\sum _{i=\pi (n ) +1}^{\pi (n+1 ) } \sum _{j=1}^{ {\bigl\lfloor\frac {\ln ( n ) }{\ln ( p_{{i}} ) }}\bigr\rfloor +1} \Bigl\lfloor {\frac {n}{{p_{{i}}}^{j}}} \Bigr\rfloor \ln ( p
 
holy latex
 
12:06 PM
What a bunch of random looking sums.
 
$$\sum _{i=\pi (n ) +1}^{\pi (n+1 ) } \sum _{j=1}^{ {\bigl\lfloor\frac {\ln ( n ) }{\ln ( p_{{i}} ) }}\bigr\rfloor +1} \Bigl\lfloor {\frac {n}{{p_{{i}}}^{j}}} \Bigr\rfloor \ln ( p_{{i}}) -\sum _{i=\pi (n ) +1}^{\pi (n+1 )} \sum _{j=1}^{\pi ( {\lfloor\frac {n}{i}} \rfloor ) } \Bigl\lfloor\frac{ \ln ( {\frac {n}{i}} ) }{ \ln
p_{{j}} } \Bigr\rfloor \ln ( p_{{j}}
) \neq 0 \Rightarrow\sum _{i=\pi (n ) +1}^{\pi (n+1 ) } \sum _{j=1}^{ {\bigl\lfloor\frac {\ln ( n ) }{\ln ( p_{{i}} ) }}\bigr\rfloor +1} \Bigl\lfloor {\frac {n}{{p_{{i}}}^{j}}} \Bigr\rfloor \ln ( p_
 
$c_7=0$ $c_8=\frac{1}{16}$, $x_7=?$, $x_8=0$, $y_7=16y_8$, $y_8=\frac{1}{16}y_7$
 
you are missing two equations
that you have written before
 
We just have $c_{2n}=0$, $c_{2n-1}=\frac{1}{2^n}$, $x_{2n}=0$, $y_{2n}=\frac{1}{2^n}y_{2n-1}$. Right?
 
no we also have $c = x+y$
 
12:08 PM
Ahhh...
Right.
 
did Adam say that $x \neq 0 \implies x \neq 0$ ?
 
well I can show you how ive got there tobias but from experience when people say something along the lines of that they arnt going to be helpful, and there is no point posting an actual question because its more of a series of conclusions the begin with a couple of PNT identities, one of them is the first one they use in the proof of Bertrand's postulate it's pretty long winded so im just waiting for me to get stuck on something significant really
 
Don't we just have that $x_7=c_7-y_7=\frac{1}{16}-16y_8$ and $x_8=c_8-y_8=-\frac{1}{16}y_7$ because of $y_{2n}=\frac{1}{2^n}y_{2n-1}\Leftrightarrow y_{2n-1}=2^ny_{2n}$?
 
nope only a very minor difference mercio that's why I find it very interesting, it can almost definitely be reduced to a much simpler statement but I need to get coke and food
the upper bound for the second double sum on the left hand side is $\pi(n+1)$ and on the right hand side of the arrow it's $n-1$
 
Well we have that $x_8=0$ so $y_7=0$. Right?
 
12:15 PM
I don't think you have copied the defintion of $c$ well enough
 
Thus $x_7=c_7=0$.
 
so what is $c_8$ ?
 
$\frac{1}{16}$
 
and what was $x_8$ ?
 
12:16 PM
so then what is $y_8$ ?
 
$\frac{1}{16}$
 
yes, now can you also deduce $y_7$ ?
don't you have an equation relating $y_7$ and $y_8$ ?
 
$y_8=\frac{1}{16}y_7\Leftrightarrow y_7=1$
 
yes
and finally, $x_7$ ?
 
$-1$
 
12:19 PM
yes
 
oh maybe this one is relevant also $$\sum _{i=1}^{\pi (n ) } \sum _{j=1}^{ {\bigl\lfloor\frac {\ln ( n ) }{\ln ( p_{{i}} ) }}\bigr\rfloor +1} \Bigl\lfloor {\frac {n}{{p_{{i}}}^{j}}} \Bigr\rfloor \ln ( p_{{i}}) -\sum _{i=1}^{n-1} \sum _{j=1}^{\pi ( {\lfloor\frac {n}{i}} \rfloor ) } \Bigl\lfloor\frac{ \ln ( {\frac {n}{i}} ) }{ \ln
p_{{j}} } \Bigr\rfloor \ln ( p_{{j}}
) =0$$
 
so this shows that there are only one possibility for $x_7,x_8,y_7,y_8$
 
Indeed.
 
so now you should redo this but try to determine $x_n$ and $y_n$ forall $n$
instead of just $7$ and $8$
 
Okay. It has to lead to some contradiction. Otherwise $c=x+y$ is valid and $c\in X+Y$.
 
12:21 PM
the contradiction will come later
 
Okay.
 
I don't think I can comprehend a sum that has so many floor function terms in it
but the liberal use of ln, suggest the relative growth of the terms might be important
 
yes I agree, I don't have enough incentive to bother looking in that latex in detail to get what that is saying
 
And... curse prime numbers, I still yet to comprehend these even though I now have a pretty solid idea on what $G_{\delta}$ sets, which is some of the most unvisualisable mathematical sets in existence, looks like in general
 
o..o'
 
12:25 PM
but I save that lengthly rambling later in another room
For now, there are only two important things I need to sort out:
1. Code the dynamic zoom in the number plotter so I can get to the 4th factorial decimal place of a liouville number
2. Organise the introduction of the thesis so at least I can call a day for the introduction
 
Hello @CaptainAmerica16. Are you 16?
 
@JasperLoy Boi
 
I see. I wish I could go back to when I was 16. =)
 
We have $y_{2n}=\frac{1}{2^n}$, $y_{2n-1}=1$, $x_{2n}=0$, and $x_{2n-1}=-1$.
 
@JasperLoy I hope I feel that way in the future. Rn life is really weird...
 
12:36 PM
yes
so if $x$ were in $X$ and $y$ were in $Y$, you would have $c \in X+Y$
 
Yes.
 
is $x$ in $X$ ?
 
yes
 
are you sure ?
 
Oh! It needs to converge, right?
Hold on...
Shouldn't it be? We only require that $x_{2n}=0$ for all $n\geq 1$ for the $x=(x_n)_{n\geq1}\in E$ where $E=\ell^1$.
 
12:40 PM
is $x$ in $\ell^1$ ?
 
No.
 
so there you have your contradiction
 
Okay.
 
if you had $E'$ be the set of all sequences
and defined $X'$ and $Y'$ in the same way replacing $E$ with $E'$
then $E'$ would be the direct sum of $X'$ and $Y'$
 
Is that why we need to consider the closure of $X+Y$ to obtain $E$?
I'm a bit unsure how closure works in sequence spaces.
 
12:42 PM
(every sequence is the sum of a sequence $x$ with $x_{2n}=0$ and a sequence $y$ with $y_{2n} = y_{2n-1}/2^n$)
(in a unique way)
well if $X+Y$ was $E$,
since $c$ is in $E$ you owuld have $c \in X+Y$
in fact this shows that $X+Y$ can't be $E$
because $c$ is in $E$ but not in $X+Y$
 
Right.
What's the difference between $X+Y$ and $\overline{X+Y}$? Because, we have that $X+Y$ is dense in $E$.
 
well in question 1 you showed that the closure of $X+Y$ was $E$
and in question 2 you showed that $X+Y$ was not $E$
:s
 
Right.
 
@Tobias I'll be there for my Master but I won't physically be in Bonn until next week
 
12:46 PM
Part 3 looks most exciting.
 
@Oskar sure, what is not clear in my answer?
 
Thanks for the help, Mercio.
 
@JasperLoy
 
@AlessandroCodenotti: I just don't see how you get $\overline{y}=x_1,x_1,2x_2,x_2,4x_3,x_3,\dots$ from $y_{2n}=x_{2n}$ and $y_{2n-1}=2^n y_{2n}$. Just insert $n=1$. Okay. Then we have $y_1=2y_2$ and $y_2=x_2$, but we have $y_1=x_1$ and $y_2=x_1$. Why?
 
Hmm, the indices might be off by one, that's not really vital, the only important thing about $\overline{y}$ is that it agrees with $\overline{x}$ in all the even positions
 
12:54 PM
Okay! Yeah. That's what I thought.
I also don't really know why $\overline{c_{00}}=\ell^1$. I suck at topology. (That's why I'm trying to get better. :))
 
I wrote in the answer how to approximate any element of $\ell^1$ with a sequence in $c_{00}$, what exactly is giving you problems?
 
Oh! No. Never mind! I get it.
Sorry to bother you.
 
Oh, no problem, I like functional analysis
 
Me too. I just have to get better at topology. I obviously struggle with elementary problems because of it.
 
Chapter 3 of Brezis requires some more topology since you consider more topologies on the same space (and they're not all metrizable)
 
1:06 PM
What is a "radial isometry" and why is it called that
 
I'm trying to prove that $(1+s)^n\geq 1+ns$ for $s>-1$, $n\in\mathbb{N}$. With recurrence, I proved it. But I'm trying to use the binomial expansion. The first two terms are indeed $1+ns$, so the goal is to show that the rest is positive or zero.
 
(the one on the weak and weak* topologies, I think it's the third but I'm not sure actually)
 
Alright! Do you think that Brezis is a good book to work with?
 
So it boils down to show that $\binom{n}{2}s^2+\binom{n}{3}s^3+\dots+\binom{n}{n-1}s^{n-1}+\binom{n}{n}s^n\geq 0$ for $s>-1$, $n\in\mathbb{N}$
For $s>0$, it's trivial, the problem is to show for $-1<s<0$.
 
1:31 PM
In the class, pairing them was tried but we didn't succeed.
Ah, I found it in the proof wiki.
 
@AbdullahUYU how about showing that $(1+s)^n - ns -1$ has a single minimum at $s = 0$ where its value is $0$
 
@AlessandroCodenotti Cool. I will be there for a conference at the end of November (well, at MPI, and I am not entirely sure what the relation is between that and the university).
 
2:06 PM
So, I hit the "approve" button on this suggested edit, but I'm not sure if I should have. math.stackexchange.com/review/suggested-edits/1075125
 
@AkivaWeinberger Would a radial isometry just preserve distances to the origin?
 
The edit completely rewrites the answer, although the new answer is saying exactly the same thing as the original answer.
I also think that the new answer is clearly better than the original answer.
But since it's so different from the original author's words, it seems like it really should have just been posted as a separate answer.
...although I just realized that the question is closed as a duplicate, and has been closed since before the edit was suggested.
 
2:23 PM
I’m in London now.
 
laaaandaaaan
 
@TobiasKildetoft I read half the course notes of group rep that I half-attended last semester
and I felt like I had never attended the course
 
Hi
I'm on a train
 
@ÍgjøgnumMeg luhn-duhn
 
@Leaky Londinium
 
2:27 PM
@TobiasKildetoft cool, I'm not sure about their relationship either though
 
Question: What is the coarsest topology on $\mathbb{R}$ such that every finite subspace of $\mathbb{R}$ has the discrete topology?
The only one I can think of is the discrete topology itself.
Maybe a topology where every infinite subset is open?
 
I think the cofinite topology works
 
Coarsest means we want lots or few open sets?
 
@Rithaniel with subspace you mean subset? Then the cofinte topology is the coarsest
 
So, every infinite subset, since we're working in $\mathbb{R}$
Coarsest is fewest. Finest is most.
What about the opposite end? The finest for which every finite subspace has the trivial topology?
 
2:36 PM
@Akiva the more open sets you have the more restrictive the statement "this net converges to a point" becomes, is finer or coarser a better description for this?
 
I dunno, normally those words mean the size of grains of sand or something, don't they?
 
@Rithaniel the discrete topology is the finest topology in existence and has this property --- nevermind you said trivial, not discrete
@Akiva a fine filter can detect more features than a coarse filter
 
Yeah, it'd be trivial to show the finest discrete topology (woo, puns!)
 
The restriction of a topology to a subset is trivial if every open set either contains the subset or does not intersect it. If you have an open set in R that is not R or the empty set, then there are points both inside and outside it. Take two such points and the restriction of the topology to those two is not trivial.
This means that the only topology trivial on finite sets is the trivial topology.
 
Oh, really? That's an interesting trait of triviality.
 
3:00 PM
Hi! Anybody here who knows about complex multiplication on elliptic curves?
if yes, any ideas on this question? mathoverflow.net/questions/301476/…
 
Unfortunately I have doubts this is the right place to find the necessary experts
 
Mike, where would you look for the necessary experts?
 
3:16 PM
o..o
 
In an algebraic number theory group?
Asking it on MO was probably the move to make. Not much more to do.
 
3:59 PM
@LeakyNun Well, if you only half-attended, then you did not really attend it
 

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